Activated carbon filter

ABSTRACT

A BONDED CHARCOAL FILTER AND A METHOD FOR MAKING THE SAME ARE DISCLOSED. THE FILTER CONTAINS FINE POWDERED ACTIVATED CHARCOAL AND A THERMOSETTING OR THERMOPLASTIC RESIN BINDER, AND IS A SELF-CONTAINED DISPOSABLE FILTER HAVING VOIDS IN SUFFICIENT AMOUNT TO PROVIDE A PRESSURE DROP OF ABOUT 0 TO ABOUT 0.5 INCH OF WATER AT A FACE VELOCITY OF UP TO APPROXIMATELY 450 FEET PER MINUTE. THE METHOD INCLUDES MIXING THE DRY INGREDIENTS AND SUFFICIENT WATER,   CHARGING TO A MOLD, APPLYING SUFFICIENT PRESSURE, AND CURING THE RESULTING FILTER.

Oct, 12, 1971 HOLDEN 3,611,678

ACTIVATED CARBON FILTER Filed Oct. 5, 1968 3,fi11,fi78 Patented Get. 12, 1971 3,611,678 ACTllVATED CARBON FILTER Herbert K. Holden, Bon Air, Va, assignor to American Filtrona Corporation, Richmond, Va. lFiled Oct. 3, 1968, Ser. No. 764,788 Int. Cl. 301d 53/04 U.S. Cl. 55-387 9 Claims ABSTRACT 015 THE DllSCLOSURE A bonded charcoal filter and a method for making the same are disclosed. The filter contains fine powdered activated charcoal and a thermosetting or thermoplastic resin binder, and is a selfcontained disposable filter having voids in sufficient amount to provide a pressure drop of about to about 0.5 inch of water at a face velocity of up to approximately 450 feet per minute. The method includes mixing the dry ingredients and sufficient water, charging to a mold, applying sufficient pressure, and curing the resulting filter.

This invention relates to filters, and more particularly to filters for removing odors from air. More specifically, this invention relates to filters of the type used to remove odors from air by having the air to be treated circulated through the same.

It is well known in the treatment of air to remove undesirable odors by means of filters of one type or another. It has been found that certain adsorbent materials such as finely divided activated carbon are particularly useful for this purpose. It is common practice to place a canister of loosely packed activated charcoal in a confined space from which odors are to be adsorbed, for example in refrigerators. In order to remove odors from the air in larger, unconfined areas, some means must be provided to circulate the air through the adsorbent material. To this end, devices have been used which consist merely of a fan in operable relationship with a canister of adsorbent material. More commonly, however, the adsorbent material is used in conjunction with a device whose primary function is to treat the air in some other manner. Such devices include air dehumidifiers as well as air conditioners for either cooling or heating the air. Another such device is the air circulating system of a frostfree refrigerator.

Such devices as air conditioners are particularly adaptable to serving the second function of passing the air through an adsorbent material. The use of a tubular canister of adsorbent material, however, is particularly unwieldly in air conditioners. Other means of containing the finely divided adsorbent material have been tried, but have all suffered serious drawbacks. Flat, rectangular confining means have been used, but these are expensive and clumsy. Attempts have been made to bind the adsorbent material 'with a resin of one type or another, but these attempts have resulted in extremely fragile filters which possess certain disadvantages common to all types of filters, namely poor adsorption and high pressure drop, thereby resulting in a loss of efficiency. Further, these filters, due to their fragility, require some sort of a container to provide the mechanical support which they inherently lack. Such a mechanical support invariably leads to further loss of efficiency through pressure drop. Due to the high cost of manufacture of these filters, they suffer the further disadvantage of not being disposable.

Accordingly, it is a primary object of the present invention to provide a filter which has good adsorbtivity, is self-contained, is disposable, and has low pressure drop.

More specifically, it is an object of the present invention to provide a bonded activated carbon filter which is free of the aforementioned and other such disadvantages.

Another primary object of the present invention is to provide a filter which can be easily manufactured at low cost and which can be utilized in existing air conditioning equipment.

Still another object of the present invention, consistent with the foregoing objects, is to provide a filter with a honeycomb type structure which is simple to install and remove for replacement.

This invention will be better understood, and objects other than those set forth above will become apparent, when consideration is given to the following detailed description of the preferred and illustrative embodiment of the invention and to the annexed drawing.

Accordingly, to satisfy the objects of the present invention there is provided an adsorptive, bonded activated carbon, self-contained, disposable, low pressure drop filter and a method for making the same. The filter contains activated carbon and a thermos-etting or thermoplastic binder and is molded and cured. The filter is essentially a grid of circular holes of such a size that the ratio of void to filled face area is between 60:40 and 40:60.

The filter is produced by mixing from approximately 50 to approximately percent activated carbon with approximately 50 to approximately 10 percent by weight of resin. Sufiicient water for processing, preferably from about 24 to about 40 percent of the total mixture is added and mixed in. The mixture is charged to a pin mold where it is compressed before discharging and curing. The compression force applied by the mold is preferably in the range of about 20 to about 50 tons. This force is achieved by applying a pressure of at least about 250 psi. The curing times and temperatures are, obviously, dependent upon the particular resin used for the binder. It has been found that for most resins a temperature range of from about 225 F. to about 450 F. for a time period of from about 2 hours to about 15 minutes is satisfactory.

While in the foregoing discussion the method of the present invention has been set forth in terms of thoroughly mixing the dry ingredients before adding the water, this sequence of steps is not necessarily critical. Equally good results are obtained, for instance, by slurrying or emulsifying the resin in the water before mixing the same with the adsorbent material. The necessary condition, of course, is that the ingredients be thoroughly mixed before charging them to the mold.

In the drawing:

FIG. 1 is a perspective view, partially broken away, of a filter according to this invention; and

FIG. 2 is a vertical cross-section taken on the line 22 of FIG. 1.

Referring to the drawings, there is shown in FIG. 1 a filter, generally designated by the numeral 10, made according to the present invention. The filter 10 comprises a finely divided adsorbent substance, preferably activated charcoal, and a resin binder. The bonded charcoal block 12 has a plurality of holes 14 therein such holes 14 being preferably circular holes. When considering either of the opposed faces of the block 12, the ratio of void to filled face area is between 60:40 and 40:60, such a construction giving a pressure drop of approximately 0.1 inch of water at 350 feet per minute face velocity, which is an acceptable pressure drop for a filter to be used in conjunction with an air conditioning unit. This void to filled surface area ratio is achieved in the preferred embodiment by providing holes 14- which are approximately 0.40 inch in diameter and are spaced approximately 0.50 inch on center. The peripheral holes 16 are spaced approximately 0.75 inch on center from the edge of the filter. It can be readily seen, that the ratio of the holes to the filled surface area in which they are contained, excluding the marginal area, is approximately 50:50 in this embodiment. This is the preferred ratio which is approached by the total surface area of the filter as the overall dimensions get larger.

Having considered certain physical characteristics of the filter of the present invention, attention is called to the method of producing the same. Generally, the method includes the steps of mixing the activated carbon, a binder and sufficient water for processing. After thorough mixing, the mixture is charged to a pin mold and a pressure of at least about 250 p.s.i. is applied. The compressed piece is discharged from the mold and cured. The curing time and temperature is generally dependent upon the type of resin being used, but usually falls in a range of about 225 F. to about 450 F. for about 2 hours to about minutes. The binder can be either a thermosetting or a thermoplastic resin, with thermoplastic resins being preferred. The proportions of activated carbon to resin in the dry mix are preferably from about 50 to about 90 percent by weight of activated cabon and from about 50 to about 10 percent by weight of resin. The preferred composition, using a thermoplastic resin, contains 30 percent resin and 70 percent activated carbon in the dry mix.

The absorbent material used in this invention is preferably finely powdered activated carbon, although a homogeneous blend of granular carbon whose maximum size is A; the wall thickness between the void holes with powdered carbon can be used as long as there is suflicient powdered carbon to form a solidly filled bonded structure. Of course, the proportions of carbon to resin remain in the same range even when using a blend of granular carbon with powdered carbon. Typical properties of a suitable carbon are as follows: iodine number, min. 900; density, min. 0.25 g./cc.; and mesh size, max. 20. The carbon used in the following examples has approximate properties as shown in Table I.

TABLE I Properties:

Iodine number, min.: 1100- Density, g./cc., min 0.40

Mesh size: 80X 325 Pore volume, cc./g., min.: 0.65-0.90

Surface area, m. /g., min.: 1200 1 Density is normally determined on larger size particle material than S0 325 as the densities determined on this size are very erratic. The typical densities of two suppliers material is 0.45 g./cc.

Typical thermosetting resins useful in this invention are the phenolic resins which are short flow, fast cure, single stage phenolics. The short flow, approximately mm., short cure, approximately seconds, thermosetting resins are particularly useful, although longer flow and longer cure materials can be used. The following Examples l-l were carried out using a void to filled face ratio of approxl mately :50.

EXAMPLE 1 A formulation was made containing five pounds of 80X 325 mesh carbon, 1.8 pounds of short flow, fast cure, single stage phenolic resin and 3.2 pounds of water. The ingredients were premixed and intimately blended. A pin mold was then filled by volume and 50 tons pressure was applied. The compressed carbon piece, or green mold, was discharged from the mold and cured for 30 minutes at 300 F. The finished filter was approximately 4 inches square by approximately /2 inch thick and weighed 59.2 grams bone dry. After room conditioning for 24 hours, it picked up 2.4 percent moisture. The bonded dry filter was composed of 73.5 percent carbon and 26.5 percent phenolic resin. Its adsorption etficiency, compared to that of samples made as described in the following examples, is shown in Table II hereinbelow.

While the thermosetting resin is extremely useful in this invention, it has been found that filters bonded with thermoplastic resin have greater resiliency and flex strength and are, therefore, tougher and more acceptable relevant to strength-handling properties. The use of thermoplastic binder, however, is limited to some extent by the temperature of the use environment. The thermoplastic binders usually soften at temperatures above about 200 to 250 F., but are, nevertheless, preferred in this invention. The thermoplastic bonding agent can comprise any thermoplastic material, such as polyhydrocarbons having from 2 to 10 carbon atoms, preferably polyolefins, e.g.,- polyethylene and polypropylene, other thermoplastic such as vinyl acetate, other vinyl homopolymers and copolymers, plasticized cellulose acetate, Artrite (a thermoplastic polyester resin), Escorex (a fine powdered petroleum hydrocarbon resin), and combinations of any of the above. The best results are obtained, however, when the bonding agent is a polyolefin, the prefererd polyolefins being polyethylene, polypropylene, or combinations thereof.

EXAMPLE 2 A mixture was made of 47.6 parts of activated charcoal and 20.4 parts of a low density thermoplastic resin with sufficient water for processing. A mold was charged as in Example 1 and a compression force of between about 20 and about 50 tons was applied. The compressed piece was discharged and cured at about 340 F. for about 45 minutes. The composition of the resulting filter, on the dry basis, was about 30 percent binder and about percent carbon.

EXAMPLES 3-15 The same procedure was followed using the proportions shown respectively in Table II.

TAB LE II Carbon Resin Parts Percent Type 1 Parts Percent Example:

3 47. 6 70 TP 20. 4 30 4 54. 5 TP 13. 5 20 5 54. 5 80 TP 13. 5 20 6- 54. 5 80 TP 13. 5 2 0 7 54. 5 80 TP 13. 5 20 8. 57. 8 TP 10.2 15 9. 57. 8 85 T1? 10. 2 15 10 46. 2 68 TS 21.8 32 11 46. 2 68 TS 21.8 32 12. 50 73. 5 TS 18 26. 5 13- 50 73. 5 TS 18 26. 5 14 50 73.5 TS 18 26. 5 15. 50 73. 5 TS 18 26. 5

l TP=therrnoplasite; TS=thermosetting.

The filters of Examples 1 through 15 were then tested to determine the relative adsorption efficiency by carbon tetrachloride adsorption. The basis for the test was unbonded carbon whose adsorption eificiency was considered to be percent. Each sample was run for 72 hours. The results are shown in Table III.

Thus, it can be seen that the relative adsorption efficiency in each case is sufiiciently high to produce satisfactory results, with the thermoplastic bonded filters having a somewhat greater adsorption efficiency. Each of the filters produced essentially no pressure drop at 300 feet per minute face velocity and no more than 0.3 in. of water at 350 feet per minute face velocity. The mechanical strength was satisfactory in each case, with the filters of Examples 2 and 3 exhibiting the most desirable characteristics.

EXAMPLE 16 A filter was made using 23.8 percent 80x325 mesh powdered charcoal, 23.8 percent 20x50 mesh granulated charcoal, 20.4 percent thermoplastic resin, and 32 percent water. Theprocedure of Example 2 was followed. The filter exhibited satisfactory characteristics.

EXAMPLE 17 Example 1 is repeated with a void to filled face ratio of 40:60.

EXAMPLE 18 Example 1 is repeated with a void to filled face ratio of 60:40.

Essentially the same results as with Example 1 are obtained with Example 17 and Example 18 but preliminary indications are that the pressure drop increases above the desired limits if the ratio of Example 17 is increased, and the strength properties of the filter decrease beyond acceptable limits if the ratio of Example 18 is decreased.

Thus, it can be seen that the objects set forth at the outset of this specification have been successfully achieved. Moreover, while there are shown and described present preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto, but may be otherwise variously embodied and practiced within the scope of the following claims.

What is claimed is:

1. A self-contained, disposable self-supporting, low pressure drop filter comprising a bonded activated carbon structure, said structure comprising a grid with a plurality of through-openings forming voids, said filter having a ratio of void to filled face area of between 60:40 and :60, said filled face area being that area enclosing the voids.

2. A filter as defined in claim 1, wherein the ratio is selected to provide a pressure drop across said filter at an air fiow of 350 feet per minute face velocity less than about 0.3 inch of water.

3. A filter as defined in claim 1, wherein the voids are annular.

4. A filter as defined in claim 1 wherein said activated carbon is bonded with a thermosetting resin.

5. A filter as defined in claim 4, wherein said resin is a phenolic-type resin.

6. A filter as defined in claim 1, wherein said actvated carbon is bonded with a thermoplastic resin.

7. A filter as defined in claim 6, wherein said resin is a polyolefin.

8. A filter as defined in claim 6, wherein said activated carbon is present in from about to about 90 percent and said resin is present in from about 50 to about 10 percent by weight of the dry mix.

9. A filter as defined in claim 1,. wherein said activated carbon has the following approximate properties:

Iodine number, min 900 Density, g./cc., min. 0.25 Mesh size, max. 20

References Cited UNITED STATES PATENTS 1,966,553 7/1934 Kropp 387 X 2,933,455 4/1960 Doying 210-502 UX 3,474,600 10/1969 Tobias 210-506 X FOREIGN PATENTS 228,837 2/1925 Great Britian 55387 REUBEN FRIEDMAN, Primary Examiner 

